This invention relates to papermakers' fabrics and especially to papermaking felts for the press section of a papermaking machine.
In the conventional fourdrinier papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
After leaving the forming medium the somewhat self-supporting paper web is transferred to the press section of the machine and onto a press felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated, generally cylindrical rolls to remove still further amounts of water therefrom.
In general, the press felts in the press section are used in papermaking machinery to support the moist, freshly formed paper web as it encounters a variety of rolls to extract water from the moist paper web. In addition to serving as a support for the moist paper web, the press felt serves as a receptacle for the water removed from the paper sheet. Preferably, the press felts are woven so as to have relatively large open areas or voids which will enhance their water-conveying capabilities until the water is removed from the felt upon passage over a suction box or other drainage device. The press felt normally has a conveyer belt-like shape and during the various operations previously mentioned, a large amount of water is built up in the press felt which is removed by suction or various other drainage devices, usually after the paper web and press felt are no longer in direct contact.
Paper sheet dewatering, sheet surface smoothness, sheet pick-up and sheet carrying, as well as a variety of other performance characteristics, are sensitive to felt design. In addition, it has been observed that the physical characteristics of felts change during their useful operating life. Generally, the originally round or irregularly shaped surface fibers tend to flatten out during operation. As surface fibers flatten, the area of interface between felt and paper increases, leading to greater adhesion between felt and sheet and therefore to improved sheet pickup and sheet carrying performance. In addition, the increased sheet/felt interface area provides a more effective sheet support in the press section leading to more efficient sheet dewatering.
In the press section of the machine, it is also advantageous to remove as much water as possible from the paper web without causing any damage thereto, such as by crushing. Crushing occurs when an excessive amount of water is removed from the paper web, the excessive water then accumulating at the ingoing side of the nip, not carrying through the nip, and moving in a direction counter to that of the moving paper web. Accordingly, care should be taken to ensure that there is sufficient void volume present in the area of the press nip (either in the papermaking fabric or in the press rolls) such that as the paper web undergoes compaction and compression, all of the water expressed therefrom will be carried through the nip.
Therefore, one object of the present invention is to provide an improved papermakers' fabric for use in the press section of the papermaking machine.
Another object of this invention is to provide an improved papermakers' felt having increased void volume and incompressibility for good wear characteristics.
Another object of the present invention is to provide an improved dewatering arrangement in the press section of a papermaking machine wherein relatively large quantities of water may be removed from a paper web without crushing or marring the surface thereof.
A further object of the present invention is to provide means for increasing the amount of water removed from a paper web in the press section of a papermaking machine.
Still another object of the present invention is to provide an improved papermaking press felt which improved water flow and compaction resistance characteristics.